No abstract

Red mud (RM) is a strongly alkaline residue generated in enormous amounts worldwide from bauxite refining using the Bayer chemical process. RM is composed mainly of Fe, Ti and Al oxides and hydroxides, but it also contains an array of trace metals and metalloids at different concentrations. The purpose of this paper is to assess the potential mobility of metals in RM, with special emphasis on pH effect. The 'operational' distribution and leachability of metals within/from RM was studied by applying a sequential extraction procedure (SEP) and several leaching tests (rapid titration, equilibration acidification, batch leaching with acetic acid and also the toxicity characteristics leaching procedure (TCLP) and the DIN 38414-S4 procedures, used as reference methods) carried out at different pH, solid/liquid ratio, extraction period and type of acid (HCl or acetic acid). Chemical analysis showed that, in addition to the major metals Fe, Al and Ti, RM contains several trace metals, some of them (Cr, Cu and Ni) in concentrations exceeding the regulatory limits. SEP showed that a majority of the metals in the RM (between the 32.2±8.5 for Cd and 95.3±0.4% for Ni) were found in the residual fraction, suggesting that they are not readily mobile under normal environmental conditions. Leaching tests performed at different pH showed that a significant fraction of the metals is mobilised from RM only under very strong acid conditions (pH<2), whereas Al is released in considerable amounts at pH<5.3. Among the trace metals, Cr requires special attention because of its relative high concentration in RM and the higher concentrations of this metal mobilised at low pH. The leaching tests using acetic acid showed that the standard TCLP largely underestimates the release of trace metals from RM, and therefore it is not advisable to evaluate the actual potential leaching of trace metals from this residue.

The utilization of biosurfactants for the bioremediation of contaminated soil is not yet well established, because of the high production cost of biosurfactants. Consequently, it is interesting to look for new biosurfactants that can be produced at a large scale, and it can be employed for the bioremediation of contaminated sites. In this work, biosurfactants from Lactobacillus pentosus growing in hemicellulosic sugars solutions, with a similar composition of sugars found in trimming vine shoot hydrolysates, were employed in the bioremediation of soil contaminated with octane. It was observed that the presence of biosurfactant from L. pentosus accelerated the biodegradation of octane in soil. After 15 days of treatment, biosurfactants from L. pentosus reduced the concentration of octane in the soil to 58.6 and 62.8%, for soil charged with 700 and 70,000 mg/kg of hydrocarbon, respectively, whereas after 30 days of treatment, 76% of octane in soil was biodegraded in both cases. In the absence of biosurfactant and after 15 days of incubation, only 1.2 and 24% of octane was biodegraded in soil charged with 700 and 70,000 mg/kg of octane, respectively. Thus, the use of biosurfactants from L. pentosus, as part of a well-designed bioremediation process, can provide mechanisms to mobilize the target contaminants from the soil surface to make them more available to the microbial population.

The interacting effect of pH, phosphate and time on the release of arsenic (As) from As-rich river bed sediments was studied. Arsenic release edges and kinetic release experiments (pH range 3-10), in the absence and presence of phosphate, coupled with sequential extraction procedures, SEM/EDX analyses and geochemical calculations, were carried out to evaluate As remobilisation and to elucidate the mechanisms involved. The results showed that As release underwent pronounced kinetic effects, which were strongly influenced by pH and phosphate. Remobilisation of As after 24 h was low (between *1 and 5%) and varied slightly with pH, whereas alkaline conditions generally promoted As remobilisation after 168 h, with up to 12-21% of total As released. The results showed that depending on the pH and sediment considered, the release of As increased dramatically after *48-72 h, suggesting that different processes are involved at different reaction periods. The addition of phosphate (1 mM) increased both the amount of As released (between 2 and 8 times) and the rate of As release from the sediments within the entire pH range (3-10) and period (168 h) studied. Moreover, in some cases, it also affected the shape of the As release edges and kinetic profiles. The similarities in the release profiles and the positive correlations between As and some sediment components, especially Fe and Al hydroxides, and organic matter-which appears to play a key role at high pHsuggest that As release from the studied sediments may be associated with solid phase dissolution processes under both acid and alkaline pH, whereas desorption plays a key role in the short term and at natural pH conditions, especially in the presence of phosphate, which acts as an As-displacing ligand. Evaluation of As mobility based on short-time leaching experiments may seriously underestimate the mobilisation of As from sediments.

A B S T R A C T : The adsorption and desorption of Hg(II) by humic acid (HA) previously adsorbed on kaolin was studied. In the range of HA concentration investigated (0.0 -26.9 mg g -1 ), the Hg(II) adsorption capacity of kaolin at pH 4 is enhanced by the presence of HA. For the complexes with the highest HA concentration and for low Hg(II) initial concentrations, adsorption was lower, i.e. as HA concentration on the complexes increases, Hg(II) equilibrium concentration also increases. This behaviour is due to the increasing presence of dissolved organic matter as the HA concentration on the complexes increases. The dissolved organic matter is able to form a soluble complex with Hg, thus decreasing adsorption. Hg(II) adsorption from a 2.5610 -5 MHg(II) solution was influenced by pH. For kaolin, a pH max (pH where maximum adsorption occurs) of 4.5 was observed. At pH values >pH max retention decreased with increasing pH. This same behaviour was observed for the kaolin-HA complex containing the lowest HA concentration (6.6 mg g -1 ). For the other kaolin-HA complexes there was little effect of pH on Hg(II) adsorption between pH 2.5 and pH 6.5. The presence of HA increased the adsorption of Hg(II) on kaolin all along the pH range studied. Desorption experiments showed that the amount of Hg(II) desorbed was quite low (<1%) for all the HA and Hg(II) concentration range studied, except for the kaolin at acid pH (pH 2.5) where the Hg(II) released was >50% of Hg(II) previously adsorbed. The presence of HA dramatically reduced this percentage of desorption to values of <3%, indicating reduced risk of toxicity problems in surface and subsurface waters. The addition of Cu(II) did not favour any Hg(II) desorption, even though Cu exhibits a strong affinity for organic matter.

No abstract

Red mud (RM), a waste arising from the bauxite refining, was studied to assess its suitability as a compacted liner material. The geotechnical and hydraulic conductivity (K) characteristics of the RM, as well as the effect of desiccation, were evaluated. RM has index properties satisfying the requirements to achieve low K. The compaction-permeability studies showed that RM may be compacted to a dense homogeneous mass using a moderate compactive effort, while lowest K (61 Â 10 À7 cm/s) was achieved for RM compacted slightly (0.4-4%) wet of optimum. Shear strength test revealed high values of friction angle and cohesion for compacted RM, and its tight structure was also evidenced by its low compressibility and maximum settlement under load. Desiccation adversely affected the permeability of compacted RM, but to a lesser extent than generally observed for clays. These results suggest that RM could be potentially utilised as a low-permeability barrier to be laid down under engineered landfill sites.

We have studied the distribution and sorption behavior of phosphate and arsenate in bed-sediments of the Anllóns river (NW Spain). As a consequence of the intense gold-mining activity in the past, substantial amounts of arsenic were found in the river sediments. For phosphorus, higher concentrations were found near two sources of P pollution. Sorption isotherms were described by the Freundlich, Langmuir and Tempkin equations. In general, the sediments sorbed more P than As. The equilibrium P concentration (EPC) reveals that sediments act as a scavenger for soluble P; by contrast, equilibrium As concentration (EAC) values were high for the As-rich sediments and correlates well with total arsenic content. Amorphous Fe oxides content, organic matter and fraction of clay plus silt were the main properties of the sediments related with the sorption of arsenate and phosphate. The results obtained provide a first estimate of the sorption behavior and availability of the phosphate and arsenate anions in the sediments of the Anllóns river.